This invention relates to a method of manufacturing screws, such as self-threading screws or locking screws, and to thread-rolling dies suitable for the manufacture of such screws.
A type of screw particularly suitable for manufacture by the method and dies of the present invention is one which contains a known trilobular geometry of the thread body and with the thread having flank angles of 60.degree., but with the tips or crests of the threads having flank angles of 30.degree.. Such a screw is commonly referred to as a 60.degree./30.degree. locking screw. In a nut or threaded hole in steel, aluminum or other appropriate work material, a thread of the foregoing type secures itself by reason of the fact that the prevailing torque of the lobular thread form at the tolerance-free 30.degree. flank provides such a high prevailing torque that the thread does not loosen through vibration. However, the screw does not provide a seal against the seepage of liquids, such as water, hydraulic fluids, and the like.
Another known screw is of the type which swages its threads in a ductile workpiece material without forming chips, and is constructed in such a way that the thread is of a standard configuration in profile, namely a thread body with flank angles of 60.degree.. Typically in such a screw, the first few threads from the tip are of progressively increasing size (i.e., tapered) and merge into a trilobular holding section which, when engaged with the workpiece, has a substantial prevailing torque, but not a liquid tight seal.
Furthermore, there is a recently known screw which not only swages its own chip free thread in a workpiece, but also provides a seal between the screw thread and the thread so formed such that no liquid, even under pressure, can seep across the thread. A self-forming and self-sealing screw of the foregoing type is basically one which contains a trilobular threaded body part at the entrance end of the screw and an adjacent threaded part of circular configuration which is capable of engaging the workpiece hole without clearance so as to form the thread seal thereat.